Gums, also called hydrocolloids, are polysaccharides. Polysaccharides are polymers of simple sugar building blocks which have been in use since about 1900. Use of gums has increased throughout the century particularly in the past 40 years and today they are used in a wide variety of products and processes. Certain micro-organisms are capable of producing polysaccharides with properties differing from those of gums from more traditional sources. The best example of such microbially-produced polysaccharides is xanthan gum. More recently discovered examples are welan gum, rhamsan gum and gellan gum.
Gellan gum, first discovered in 1978, is produced by strains of the species Sphingomonas elodea [formerly Pseudomonas elodea], in particular strain ATCC 31461 [Kang, K. S. et al EP 12552 and U.S. Pat. Nos. 4,326,052; 4,326,053; 4,377,636 and 4,385,125]. Commercially this gum is produced as an extracellular product by aqueous cultivation of the micro-organisms in a medium containing appropriate carbon, organic and inorganic nitrogen and phosphate sources and suitable trace elements. The fermentation is carried out under sterile conditions with strict control of aeration, agitation, temperature and pH [Kang et al, Appl. Environ. Microbiol., 43, [1982], 1086]. When fermentation is complete, the produced viscous broth is pasteurized to kill viable cells prior to recovery of the gum. The gum can be recovered in several ways. Direct recovery from the broth yields the gum in its native or high acyl [HA] form. Recovery after deacylation by treatment with a base yields the gum in its low acyl [LA] form. Acyl groups present in the gum are found to influence its characteristics significantly.
The constituent sugars of gellan gum are glucose, glucuronic acid and rhamnose in the molar ratio of 2:1:1. These are linked together to give a primary structure comprising a linear tetrasaccharide repeat unit [O'Neill M. A., et al, Carbohydrate Res., 124, [1983], 123 and Jansson, P. E., et al., Carbohydrate Res., 124, [1983], 135]. In the native or high acyl [HA] form two acyl substituents, acetate and glycerate, are present. Both substituents are located on the same glucose residue and, on average, there is one glycerate per repeat unit and one acetate per every two repeat units. In the low acyl [LA] form, the acyl groups have been removed to produce a linear repeat unit substantially lacking such groups. Light scattering and intrinsic viscosity measurements indicate a molecular mass of approximately 5×105 daltons for [LA] gum [Grasdalen, H. et al., Carbohydrate Polymers, 7, [1987], 371]. X-ray diffraction analysis shows that gellan gum exists as a three-fold, left-handed, parallel double helix [Chandreskaran et al., Carbohydrate Res., 175, [1988], 1 181, [1988]23].
Low acyl [LA] gellan gums form gels when cooled in the presence of gel-promoting cations, preferably divalent cations, such as calcium and magnesium. The gels formed are firm and brittle. High acyl [HA] gellan gums do not require the presence of cations for gel formation and the gels formed have structural and rheological characteristics which are significantly affected by the acyl substituents. Thus the properties of [HA] gellan gels differ significantly from those of [LA] gellan gels. [HA] gels are typically soft and flexible and lack thermal hysteresis.
Typical gelation temperatures for [LA] gellan gums are in the range 30° C. to 50° C., depending upon the nature and concentration of the cations present. For purposes of this patent, gelation, set and melt temperatures are defined by measurement of the elastic modulus (G′) value of the gel in an appropriate rheometer. Conditions used are a frequency of 10 radians/second with a strain level of 1-5%. In most cases, the appropriate temperature is judged by the rate of change in the modulus value. A rapid increase with cooling is the setting temperature; a sharp drop indicates the melt temperature when heating. Frequently, the temperature where the modulus goes above or below a value of 1 Pa is used as an index. Typical gelation temperatures for [HA] gellan gums are in the region of 70° C. The high gelation temperature of [HA] gellan gum can be advantageous in some applications such as fruit fillings where it can prevent flotation of the fruit. In other applications, however, such as ready-to-eat jellies and confectionery, the high gelation temperature can be a problem with regard to pre-gelation prior to depositing.
A wide range of gel textures can be produced through manipulation of blends of [HA] and [LA] gellan gum. However, it has been demonstrated that mixtures of [HA] and [LA] forms exhibit two separate conformational transitions at temperatures coincident with the individual components [Morris, E. R., et al., Carbohydrate Polymers, 30, [1996], 165-175]. No evidence for the formation of double helices having both [HA] and [LA] molecules has been found. This means that problems associated with the high gelation temperature of [HA] gellan gum still exist in blended systems.
It has been demonstrated that treatment conditions using strong bases such as potassium hydroxide during recovery influence both the composition and Theological properties of gellan gum [Baird, J. K., Talashek, T. A., and Chang, H., Proc. 6th International Conference on Gums and Stabilisers for the Food Industry, Wrexham, Clwyd, Wales. July 1991—Edited Phillips G. O., et al, published by IRL Press at OUP [1992], 479-487]. This suggests that control of acyl content by strong base treatment during the gum recovery process can lead to a diversity of textures. To date, however, this observation has not led to such control being realized on a commercial scale. Consequently, gellan gum remains available essentially in two forms only, i.e. [HA] and [LA].
Gellan gums have a wide variety of applications in food and non-food manufacture and the provision of a range of forms in addition to the basic [HA] and [LA] forms, i.e. a range of intermediate forms, other than blends, is desirable. Such new forms of gellan gums are potentially useful in the current search for suitable alternatives to gelatin.
The texture of native gellan gum is ideal for a number of commercial food applications, including milk-based products such as puddings, coffee creamers, drinks and desserts. The rheology of gellan gum at low dosage enables it to suspend fine particles such as cocoa in milk systems. As a result of these textural characteristics, gellan gum has long been sought for use in cultured dairy products, retorted dairy products and frozen dairy products.
U.S. Pat. No. 6,602,996, incorporated herein in its entirety, describes the production of high acyl gellan gum compositions which comprises a structure having linear tetrasaccharide repeat units of glucose residues to some of which residues are attached acetate and/or glycerate substituent groups wherein the ratio of acetate substituent groups to glycerate substituent groups is at least 1.
High acyl gellan gums have been used in yogurt drink products, for fruit pulp suspension, and in retort milk beverages but with limited success. The HA gellan samples used in retort milk beverages were not tested for setting temperature or thermal hysteresis tests, only for gel strength. The polymer failed to suspend colloids reproducibly because of this in appropriate gel strength test. Moreover, such high acyl gellan gum could not be used in neutral milk beverages due to off flavor issues with p-cresol development in UHT and HTST applications. The high acyl gellan gum had poor suspension capability for cocoa in dairy or soy based systems, or fruit pulp in juice beverages, due to partial deacylation or lower set temperatures and measurable thermal hysteresis. Such high acyl gellan gum compositions are not calcium stable and tend to break down. Unless produced under appropriate conditions, the HA gellan gum contains some LA components which damage its functional properties in these systems.
Kelcogel, low acyl gellan, a commercially available product in the USA since 1993 behaves quite differently in beverages, it requires sequestrant prior to hydration, is protein and calcium sensitive and has a much narrower working range for varying calcium levels than these new high acyl gellan molecules